A femoral sizing instrument for use in a knee joint replacement procedure has a housing which includes formations (4) by which it can be located relative to the femoral posterior condyles. A cortex arm (14) has a tip (16) for engaging the anterior cortex of the femur and a sulcus arm (20) has a tip (24) for engaging the sulcus. Each of the cortex and sulcus arms can be adjusted relative to the housing to adjust the distance between its tip and the posterior condyles measured generally parallel to the anterior posterior axis. The instrument includes at least one scale (13,23,27,25) for indicating the positions of the tips of the cortex and sulcus arms relative to the posterior condyles.